The subject matter of the present invention relates to a measurement apparatus adapted to be connected between metallic housings of a wellbore apparatus for measuring the static spontaneous potential (SP) of an earth formation traversed by the wellbore.
An earth formation traversed by a wellbore contains flowing currents which are generated by a naturally occuring electromotive force (emf) of electrochemical origin present within the formation. Attempts have been made to measure this electromotive force, however, the flowing currents have interferred with the acquisition of an accurate measurement of this electromotive force.
For example, as noted in FIG. 12, a single measurement electrode placed downhole on an insulated mandrel will not accurately produce a true reading of this electromotive force and therefore a distortion will appear in the spontaneous potential (SP) measurement.
As noted in FIG. 13, a measurement electrode is disposed between two metallic housings of the downhole tool. The measurement electrode does not produce a true reading of the electromotive force. More particularly, a spontaneous potential (SP) sub is connected between metallic housings of a wellbore apparatus and the wellbore apparatus is lowered into the wellbore for the purpose of measuring the spontaneous potential of the earth formation. However, the quality of the measurement is adversely affected by the proximity of the metallic housings of the wellbore apparatus disposed on both sides of the SP sub. The SP sub includes a measurement electrode designed to measure the spontaneous potential of the earth formation. The metallic housings of the wellbore apparatus are positioned on both sides of the measurement electrode. The currents generated by the formation's emf flow in the formation. However, since the metallic housings are located on both sides of the measurement electrode, the metallic housings short circuit the flowing currents to ground potential and, in doing so, cause such currents to flow near general vicinity of the measurement electrode of the SP sub. This causes a distortion in the SP measurement. In order to reduce the above referenced distortion, the spontaneous potential (SP) sub was reconnected to the absolute bottom end of the wellbore apparatus so that no metallic housing was present on one side of the measurement electrode. As a result, since one metallic housing was not disposed adjacent the measurement electrode, the quality of the SP measurement was somewhat improved. Nevertheless, since it is always necessary to connect the SP sub to the absolute bottom end of the wellbore apparatus prior to lowering it into the wellbore, the SP measurement is still somewhat distorted.
In addition, as noted in FIG. 4, one partial solution to this problem is found in U.S. Pat. No. 2,592,125 to Doll, the disclosure of which is incorporated by reference into this specification. This partial solution is also discussed in an article entitled "Selective SP Logging", by H. G. Doll, Petroleum Transactions, AIME, vol 189, 1950, the disclosure of which is also incorporated by reference into this specification. This partial solution (hereinafter known as the "Doll Approach") involves the placement of two electrodes G and A on both sides of the measurement electrode M, and driving the A electrode with a current which is sufficient to counteract the naturally occuring flowing currents in the formation and therefore maintain the potential difference between the M and G electrodes at approximately zero potential. As a result, the flowing currents, flowing in the vicinity of the measurement electrode M, are nearly zero. Therefore, at least some of the distortion in the SP measurement, created by these flowing currents, is removed. However, another problem exists with respect to the partial solution suggested by Doll Approach. Although the A electrode is driven with a current sufficient to maintain the M-G electrode potential difference at approximately zero, the current generated by the A electrode must flow via the earth formation to a return or ground electrode located at the surface of the wellbore. In fact, the Doll Approach fails to recognize that the return or ground electrode could be the metallic housing itself. As a result, as shown in FIG. 4, a basic instability is inherent in the model of the Doll Approach. The measurement electrode does not produce a true reading of the electromotive force of the formation and therefore a distortion still appears in the SP measurement.
If the Doll Approach were utilized when the SP sub is connected between the two metallic housings of the wellbore apparatus, the return or ground potential would then be the metallic housings themselves and not the surface oriented electrode. With this new configuration, the basic instability of the Doll Approach is eliminated. With this new configuration, the SP sub could be interconnected between two metallic housings of the wellbore apparatus tool string, still maintain the M-G electrode potential difference at approximately zero, and there would be substantially no distortion in the SP measurement.